1. Field of the Invention
The present invention concerns a mold for use in continuous metal casting having a cavity opened at both ends.
2. Description of the Related Art
A continuous casting process is adapted to produce cast products such as billets, blooms or slabs continuously from molten metals and has advantages of high yield and capable of producing products with reduced number of steps.
The continuous casting process, however, has a problem in that air gaps are formed between a solidified shell of a cast product and a mold wall. The air gaps remarkably lower heat transmission between the mold and the solidified shell, give not uniform cooling for the solidified shell, form internal cracks at the corners of the cast product, which result in break-out of the cast products in an extremely case. The internal cracks at the corners of the cast product are caused by delayed cooling at the corners of the cast product, and it is considered that the cracks are formed by bending stresses (tensile components) exerted on the corners of the cast products due to uneven thickness of the solidified shell caused by difference of cooling conditions between the sides and the corners of the mold during proceeding of cooling under intermittent contact in the course of continuous withdrawal also after the formation of the air gaps in the mold.
In order to prevent occurrence of the air gaps, optimization of a taper for a cast cavity (space in the mold for forming the cast product), injection of a coolant into the air gaps and the like have been proposed for attaining effective contact between the solidified shell of the cast product and the mold wall.
For example, as a method of optimizing the taper for the mold cavity, a mold for use in continuous casting considering a hot extraction characteristic value of mold flux in the mold also used as a lubricant has been proposed (refer to Japanese Published Unexamined Patent Application No. 56-53849, which is hereby fully incorporated by reference). As shown in FIG. 4a and FIG. 4b, a taper at a shorter side 12 of a mold is determined so as to satisfy a specific condition (relation) in order to improve the cooling state at the corners of a cast product as a slab mold. The mold wall surface has a convex protruding toward a slab along the casting direction, and the taper is increased within a range from 5 cm to 10 cm in the vicinity of a meniscus position, while decreased toward the bottom (exit) so as not to increase friction between the cast product and the mold wall in the lower portion of the mold.
The patent literature describes that this constitution can eliminate the air gap caused on the shorter side of a slab mold and, further, prevent break-out of a cast product by suppression of friction between the cast product and the mold wall which increases along with increase of a casting speed, as well as can prevent break-out of the mold by drastically reducing longitudinal cracks on the surface and the inside of the corners of the cast product formed frequently so far in specific kinds of steels (high carbon steel, low alloy special steel, and the like).
Further, a mold for use in continuous casting has also been proposed, as shown in FIG. 5a, in which an additional extension 15 is disposed to a portion at an upper-half 13 of a continuous casting mold to form an enlarged transverse cross sectional portion 16 (refer to FIG. 5b), and the circumferential length of the mold is partially increased by the provision of the enlarged transverse cross sectional portion 16, thereby aligning the circumferential length of the mold with the circumferential length of the cast product upon solid shrinkage, and suppressing formation of air gaps at the corners of the mold (refer to Japanese Published Examined Patent Application No. 7-67600, which is hereby incorporated by reference).
The patent publication describes that the constitution can prevent surface defects at the corners and greatly reduce destruction or break-out of cast products tending to occur in high speed casting.
In addition, it has also been proposed a continuous casting mold for a cast product of a circular cross section having a taper of 5.0xcx9c19.0%/m in a meniscus portion considering solid shrinkage accompanied by xcex4xe2x86x92xcex3 transformation of a solidified shell just after solidification in hypoperitectic steels (0.08xcx9c0.15 mass % C) (Japanese Published Unexamined Patent Application No. 9-314287, which is hereby incorporated by reference).
Since the hypoperitectic steels have low carbon content, the structure at the initial stage of solidification shows a xcex4 phase like that pure iron, which transforms to a xcex3 phase along with proceeding of cooling. As shown by A in FIG. 2, phase transformation from the xcex4 phase to the y phase at the initial stage of solidification results a relatively large change in the specific volume and, accordingly, a taper corresponding to the solid shrinkage accompanied by the phase transformation is provided to the meniscus portion of the mold.
The patent literature describes that the disclosed continuous casting mold can prevent formation of air gaps formed by large solid shrinkage accompanied by transformation of a solidified shell from the xcex4 phase to the xcex3 phase, and formation of cracks in the cast product due to solidification delay in the solidified shell at the portion of the air gaps.
However, even by the above-mentioned method of optimizing the taper for the mold cavity, formation of the air gap at the corners of the mold wall can not be completely prevented or suppressed. This is because any of the methods described above is based on the shrinkage of the solidified shell cooled and formed in the solid state (solid phase), and the taper formed thereby for the mold cavity can not be said appropriate but forms air gaps between the mold and the cast product and, further, forms air gaps at the corners of the mold to make a delay in the cooling speed, which brings about internal vertical cracks at the corners of the cast product. That is, it is difficult to eliminate the air gaps at the corners of the mold wall conforming to the solid shrinkage of the solidified shell of the cast product including the corners of the mold wall.
In the existent mold for use in continuous casting described above, internal cracks at the corners of the cast product, particularly, internal cracks at the corners of the cast product formed in a thin chilled layer present 2 to 3 mm below the surface layer can not sometimes be prevented.
The internal cracks at the corners of the cast product are caused by cooling delay for corners of the cast product due to the air gaps inevitably formed at the corners of the mold wall described above, and the internal cracks become remarkable along with increase of the casting rate, leading to a problem that the productivity of continuous casting by increasing the casting rate can not be improved.
Furthermore, the internal cracks at the corners of the cast product increases degree of fabrication to the cast product, reduces the yield and, in an extreme case, causes break-out of the cast product, to interrupt continuous casting operation and, as a result, remarkably lowering the productivity in the continuous casting.
An object of the present invention is to provide a mold for use in continuous metal casting capable of preventing internal cracks formed in a chilled layer at the corners of a cast product and capable of coping with increasing casting rate.
The present invention has been accomplished on the basis of a novel finding that internal cracks formed in a chilled layer at the corners of a cast product are caused by formation of air gaps due to shrinkage accompanied by the phase change of molten metal supplied to the mold from a liquid phase to a solid phase in continuous casting to form a solidified shell from the molten state at the meniscus portion.
According to the present invention, in order to prevent formation of air gaps caused by the shrinkage, a restriction portion corresponding to the amount of shrinkage along with formation of a solidified shell from a molten metal near the meniscus is additionally disposed to an inner wall surface of a continuous casting mold, thereby maintaining the solidified shell of the cast product in an effective contact state from the initial stage of formation to withdrawal from the mold.